In the art of manufacturing pneumatic tires, rubber flow in the tire mold or minor differences in the dimensions of the belts, beads, liners, treads, plies of rubberized cords, etc., sometimes cause non-uniformities in the final tire. Non-uniformities of a sufficient amplitude will cause force variations on a surface, such as a road, against which the tires roll which produce vibrational and acoustical disturbances in the vehicle upon which the tires are mounted. Regardless of the cause of the force variations, when such variations exceed an acceptable maximum level, the ride of a vehicle utilizing such tires will be adversely affected.
The adverse effects of tire non-uniformity result from the non-uniformity causing fluctuations in several types of forces which are simultaneously exerted against a surface by a tire during its rotation under load. For example, a tire non-uniformity best described as the "out of roundness" of the tire, causes variations in the radial forces on a tire, which are forces exerted in the radial direction of the tire or in a direction perpendicular to its axis of rotation and non-tangential to the road surface. Further, lateral forces, which are forces exerted in the axial direction of the tire or in a direction parallel to its axis of rotation, are increased by excessive conicity, defined as one-half of the net average lateral force resulting from a non-conical shaped tire, and this causes a tire to constantly pull in one direction.
In a non-uniform tire, the radial and lateral forces exerted by the tire will vary or change during its rotation. The variations in radial and lateral force during rotation of a tire are usually caused by differences in the stiffness and/or geometry of the tire about its circumference or tread centerline. If these differences are slight, the radial and lateral force variations are insignificant and their effects unnoticeable when the tire is installed on a vehicle. However, when these differences exceed a certain level, the radial and/or lateral force variations may be significant enough to cause rough riding conditions and/or difficult handling situations. Further, as stated previously, an excessive conicity value will cause a rolling tire to pull to one side.
Consequently, methods have been developed in the past to correct for excessive force variations by removing rubber from the shoulders and/or the central region of the tire tread by means such as grinding. Most of these correction methods include the steps of indexing the tire tread into a series of circumferential increments and obtaining a series of force measurements representative of the force exerted by the tire as these increments contact a surface. This data is then interpreted and rubber is removed from the tire tread in a pattern generated by this interpretation. These correction methods are commonly performed with a tire uniformity machine, which includes an assembly for rotating a test tire against the surface of a freely rotating loading wheel. In this testing arrangement, the loading wheel is moved in a manner dependent on the forces exerted by the rotating tire and those forces are measured by appropriately placed measuring devices. When a tire being tested yields less than acceptable results, shoulder and center rib grinders are used to remove a small amount of the tire tread at precisely the location of non-uniformities detected by the measuring devices. As the tire is rotated, it is measured and ground simultaneously. In a sophisticated tire uniformity machine (TUM), such as a Model No. D70LTW available from the Akron Standard Co. of Akron Ohio, the force measurements are interpreted by a computer and rubber is removed from the tire tread using grinders controlled by the computer. Examples of machines utilizing these methods are disclosed in U.S. Pat. Nos. 3,739,533, 3,946,527, 4,914,869, and 5,263,284.
Any vibration that is generated by the grinder assembly of the tire uniformity machine is detected by its force variation measuring elements. Small quantities of vibration are acceptable because electronic filters are used to remove this extraneous noise. But when the motor bearings wear out or grind wheels are defective or improperly installed, excessive vibration will occur. Detection of this excessive vibration by the force measuring elements causes the tire uniformity machine to grind the tire tread at the wrong locations, resulting in excessive grind time, fewer tires processed, and more scrap tires. In addition, there is a need to detect the beat frequencies caused by two sources of vibration, such as the grinder motors, with slightly different frequencies and phase.
Currently, the prior art method of detecting excessive vibration is to use an external vibration analyzer with a movable accelerometer, which a technician manually locates on different points of the tire uniformity machine. Problems with this technique are that the equipment is costly, it takes several hours to complete the vibration analysis, and the resulting downtime of the tire uniformity machine is expensive. Also, since defects in the machine are usually discovered on an infrequent basis, problems are often not discovered before more costly damage results.
As discussed above and illustrated by the previously cited patents, efforts have been, and are continuously being made to more efficiently correct tire non-uniformity. Nothing in the prior art, however, suggests evaluating the quantity of extraneous vibration in the tire uniformity machine prior to mounting a tire.